1. Field of the Invention
The present invention relates to a method and apparatus for screw in-line type injection molding, and more specifically to a low-pressure screw in-line type injection molding method and apparatus for the low-pressure injection molding of urea mixed resin powder.
2. Description of the Prior Art
In the molding process utilized by most popularly marketed screw in-line type injection molding machines, the materials to be molded must be smoothly fed out of a hopper into an injection cylinder. For this purpose the particle size of the materials is usually adjusted to prevent bridging and to assure a good supply. The materials supplied are sent to the tip of the injection cylinder while simultaneously being molten or plasticized and mixed together by a rotating screw. Thereafter the screw is pushed back by the reactive force of the materials, and when a specified volume has been forwarded, a limit switch acts to stop the screw rotation. The materials are then injected into the cavity of a metal mold.
In the conventional screw in-line type injection molding machine the injection pressure and the injection velocity are closely related to each other. Depending on the respective stages of the plasticized molten or the plasticized state of the materials, the pressure and the velocity are respectively adjusted by a pressure control valve and a flow rate control valve in a hydraulic circuit. In the current practice the optimum values of the injection pressure, velocity, and time are usually set empirically. In the case of thermosetting resins, such as phenol resin and urea resin, the injection pressure is set as high as 1000-1500 Kg/cm.sup.2 and the injection velocity is set at more than 50 mm/sec for one injection shot of 20-200 g.
The pressure-hold time, defined as the duration of time during which the molten or plasticized material to be molded is held in the cavity of the metal mold under a specific pressure after being ejected from the injector nozzle, has a great influence particularly on the shrink rate of the molded product. Accordingly, the material is usually held in the mold at a pressure less than or equal to the injection pressure for a considerable length of time until the materials cease to flow back out of a sprue of the mold and the shrinkage of the molded product becomes minimal. It is said that the necessary holding time is 5-20 seconds.
Conventionally, materials of this kind can be molded by two methods one using a plunger type injection molding machine; and the other using a screw in-line type injection molding machine which operates under a high pressure.
These methods, however, have the following drawbacks. In the first method, a preheat-melting chamber is required; the machine is complicated and inferior in workability; the productivity is poor, because the materials have to be perfectly molten and in consequence the cooling time after injection into the cavity of the metal mold becomes long; and, since urea mixed resin has to be heated to a temperature over the melting point for a long time, this process not only adversely affects the properties of urea mixed resin but also contaminates the work environment with ammonia gas and cyanogen generated thereby.
In the second method using a high pressure for injection, a heavy machine is required. Due to the high pressure the metal mold must be a rugged one, and accordingly it is difficult to produce a profile containing a low-strength core made from, for example, ceramic material.
To eliminate these drawbacks, the best method is to use a screw in-line type injection molding machine with an extremely low pressure. However, the conventional technology does not produce a good molded product by injection at an extremely low pressure because an injection resistance is inherent in the injection molding machine itself.
The present invention renders it possible to make a low-pressure injection of, specifically, urea mixed resin powder, using the screw in-line type machine.